Battery Capacity and Runtime Calculator

This battery capacity calculator works out how long a battery will run and how much energy it holds, using nothing more than the numbers printed on the battery and the load you plan to put on it. Enter the battery's capacity in amp-hours, the current draw of your load in amps, and the battery's voltage, and the calculator instantly returns the estimated runtime in hours, the stored energy in watt-hours, and the load current you entered for reference. The maths is straightforward: runtime is capacity divided by current, and energy is capacity multiplied by voltage. It is useful whenever you need a quick sizing check, choosing a battery for a solar setup, a camping fridge, a drone, a UPS, or any device where you know the load but not how long the battery will last. Change any of the three inputs and the results update immediately, so you can compare different battery sizes or work backwards from a required runtime to the capacity you need to buy. Keep in mind that real batteries rarely deliver their full rated capacity once you push them hard: high discharge currents, cold temperatures and normal ageing all shrink the usable amp-hours, an effect engineers call the Peukert effect. Treat the figures here as a best-case starting point and build in a safety margin, particularly for anything safety-critical or hard to recharge in the field.

Calculate.co.nz is proud to be partnered with Premium Homes, a recognised leader in eco-friendly, sustainable, and energy-efficient homebuilding. With a dedicated team and award-winning experience, they create homes that prioritise health, comfort, and long-term performance. Their founders, Andrew and Kelly, set out to raise the standard of residential construction in New Zealand by combining practical building expertise with a clear commitment to doing things better for homeowners.